Abstract Natural killer (NK) cells kill tumors and infected cells by integrating signals from two functional types of receptors, activation and inhibitory. In the mouse, many of these receptors belong to the Ly49 family, encoded by a cluster of highly related genes, discovered by the applicant’s laboratory. The highly polymorphic Ly49 receptors and killer immunoglobulin-like receptors (KIRs) on human NK cells are examples of convergent evolution. In murine cytomegalovirus (MCMV) infection, the applicant’s laboratory previously showed that the Ly49H activation receptor is responsible for genetic resistance of C57BL/6 mice to MCMV infections. Ly49H recognizes a molecule encoded by MCMV, in a manner independent of major histocompatibility complex class I (MHC-I) alleles. However, studies from other groups suggest that in non-C57BL/6 mice, other Ly49 activation receptor alleles recognize MCMV-modified MHC-I. Moreover, still other data suggest that Ly49 inhibitory receptors can also provide protection against MCMV, depending on MHC-I alleles. It has been challenging to study these issues further due to the complexities of the Ly49 receptors, including their polymorphism and variegated expression. Nonetheless, these issues are important to understanding the role of human inhibitory KIRs and their MHC-I ligands that are also paradoxically associated with protection from viral infection. Here the applicant presents preliminary data that MCMV is controlled by NK cells in a manner independent of Ly49H but restricted to a specific MHC-I allele. Preliminary data from the applicant’s laboratory strongly support a role for a Ly49 inhibitory receptor. Thus, the Specific Aims of this proposal are to: 1) Identify Ly49 responsible for MHC-restricted resistance to MCMV; 2) Determine role of Ly49 responsible for MHC-restricted resistance; and 3) Determine how MCMV is involved in Ly49-dependent resistance. Thus, these studies will provide new insight into the function of NK cell inhibitory receptors in infection.